Method and apparatus for recovering silver from fixing solutions



F.' E. GARBUTT ET AL July 12, 1932.

METHOD AND APPARATUS FOR RECOVERING SILVER FROM FIXING SOLUTIONS FiledMarch 13. 1929 "llhplld Tha/MQW. [7gg/flaw. Fra/IME 6er-bul?. l

Patented July 12, 1932 UNITED STATES lPATENT OFFICE 'FRANK E. GVABJBUTTAND THOMAS I. INGIAN, Ol* LOS ANGEDES, CALIFORNIA, AS-

SIGNOBS TO KODAK COMPANY, A CORPORATION OP NEW YORK 'IETIOD ANDAPPARATUS FOB REOOYERING SILVER SOLUTIONS Application led Iarch 13,1889. Seriallo. 346.688.

This invention is concerned with the recovery of silver from hyposolutions used in the ixing treatment in photography. The present methodhas particular utility when 5 practiced in connection with the wellknown mechanical methods of'developing and -fixing motion picture ilms,and will therefore be described particularly with that application inview, but without necessary limi- 10 tation thereto.

The common practice of manipulating hypo solutionrin the past has beento run film through a batch of hypo until the hypo becomes saturatedwith silver, or nearly so, 15 and then to remove the silver saturatedhypo solution and replace it with a fresh batch. Silver is laterrecovered from the-saturated solution by precipitation.V l a Thispractice has several disadvantages, which will briefly be indicated. Thesilver saturation point of a hypo solution of usual concentration hasbeen found to be approximately 3 oz. of silver per gallon of solution. Abath of hypo solution has commonly been used until a concentration ofsay 1.6 oz. of silver is reached, when the solution is replaced by afresh batch. It has been found not practicable to approach the point ofsilver saturation more closely for the reason that the effectiveness ofthe hypo is continuously decreased as this saturation point is aproached, and the silver salts may not completely removed in the timethe film is allowed in the bath. Theelectiveness of the hypo'thus variescontinuously in accordance with the use of the solution, and-thesolution must-be replaced before silver salts begin to be leftundissolved in the film.-

It has been ascertained that about one gallon of hypo is carried overand lost from the fixing solution by approximately three thousand feetof film. Assuming that the silver concentration of the hypo is normallyallowed to rise to 1.6 oz. per gallon before replacement, the averageconcentration is .8 oz, per gallon. This means that .8 oz. of silver islost upon the film for every three thousand feet of lilm run through thebath.

The two major disabilities of these methods which maybe gathered fromthe above discussion are, first, a continuous variation in thee'eetiveness of the hypo on the silver salts of the film, and second, arelatively large quantity of silver lost by carrying over relatlvelysaturated hypo on the lm.

According to the present invention the film is thus maintained constant,and as the silver concentration of the solution is maintained at arelativelylow value, a relatively small quantity of silver is lostby'carrying over on the film. According to the present invention, thisresult may be accomplished, for instance, by continuously circulatingthe hypo solution of the fixing tank through a recovery tank withinwhich the silver is electrically deposited upon metal cathode plates,which are periodically removed from the tank, dried, and scaled toremove the metallic silver.

Silver is not readily recoverable from hypo solutions byelectro-deposition, however, and to reduce the silver concentration bythis method to the low value desired we have had recourse to s ecialmethods for promoting the action. e have found, for instance', that thedeposition `of silver isgreatly increased if the hypo solution isthoroughly 'tated as bypassing streams of air through 1t during' theelectrical treatment. By this means the concentration may easily bemaintained at a value as low as Say .2 oz. per gallon, at which the hypois at practically fullstrength for the fixing treatment, and at whichonly a relativel small quantity of silver is lost upon the The agitationproduced by these air streams also prevents thepreci itation of silversulfide, which otherwise orms in large.

quantities.

The chemistry relating to the electrolysis of hypo solutions -is verycomplex and need notbe comprehensively set forth for a com-A pleteunderstandingl of our invention. It is suicient to note that the depositon the cathvthe, hypo solution on the silver salts of the removablecathode plates ode of silver from the h o solution is a secondaryreaction. The li ration of sodium catlons at the cathode is a primaryreaction of the electrolysis and the liberated sodium laden hyposolution will promote theA plating of metallic Vsilver and prevent theformation of undesirable compounds such as silver sul-v lfide. Althoughthe whole electrolyte is agi- Y h tated it will be apparent that thecirculation of the hypoor f ixing solution across the lsurface -of thecathode isthe most important factor in the procurement vof a goodmetallic silver deposit on the cathode.-

The invention will best be del-Stood from the following detaileddescription of a present illustrative embodiment, reference for thispurpose bein had to the accompanying4 drawing, in whlc y Figure 1illustrates diagrammatcally the system of the invention as a whole;

Fig. 2 is a vertical longitudinal section of the `silver-recovery tank;

. Fig. 3 is a vertical transverse section, takenon line 3-3 of Fig. 2;and

Fig. 4 is a perspective view of 'one of the upon which the silver isdeposited.

We have chosen to illustrate an embodiment of the invention adapted foruse in connection with a mechanical system of developing and .fixingmotion picture This embodiment will be understood, however, toillustrate the invention by indicating merely one of its fields of use,and one of its practical embodiments therein.

Referring now to the drawing, and particularly to Fig. 1, the film 10 isshown passing continuously through developing tank 11, wash tank 12,fixing or hypo tank 13 and final wash tank 14. Pipe line 15 leads fromone end of fixing tank 13, and discharges hypo into the end ofsilver-recovery tank 16, which contains a number of anode and cathodelates across which an electrical potential 1s maintained, as by means ofD. C. generator Cr. Fixing tank 13 may conveniently be located aboverecovery tank 16 so that hypo will flow to the recovery tank without thenecessity for a pump in pipe line 15. The hypo flows from pipe 15continuously through tank 16, giving up its silver upon the cathodeplates by the action of the Yelectric current, and is discharged fromthe other end of the tank by. way of chute 17 into a tanklS.

The electrical treatment received in trough 16 tends to warm the hypo;and for the pury pose of thereafter reducing its temperature to a propervalue, cooling coils 19 for circulation of acooling medium such as brinemay be provided in tank 18.

A pipe line 20 containing a pump 21 driven by motor 22 conducts thecooled hypo from tank 18 back up to fixing tank 13.

Referring now to Figs. 2 and 3, tank 16 comprises, for instance, awooden or other insulating trough having a bottom 30, ends 31 and 32,and side walls 33. End 32 is provided with the discharge chute 17,hereinbefore mentioned. t

Side walls 33 are grooved as at 36 to receive a number of carbon anodeplates 37 evenly spaced longitudinally in the'trough. Grooves 36terminate a short distance, say an inch or so, above bottom '30, therebyproviding shoulders 38 upon which. plates 37 may rest at aVsuitable'clearance from the bottom that the hypo may circulate under theplates.. For the purpose of setting out roportions which may besuitable, but wit out intention of limiting the invention thereto, theseanode plates may be about 15 inches square, there being any number ofplates necessary to accommodate the quantity of hypo to be treated. Theanode plates are referably of carbon, as before stated, thoug some othermaterial may be found suitable. The anode plates are-all electricallyconnected together by means of a metal conducting strip 40 screwed toone side wall 33, and contact screws 41 threaded in strips 40 andpassing through the side wall to make electrical contact with the edgesof the plates, as shown in lgivenly spaced between the' carbon anodeplates are removably suspended cathode ilm. 'plates 45, upon which thesilver isto be de' posited. These `plates are of some suitable metalupon which the silver will readily deposit and which will not beattacked by the hypo solution. I have found that cathode plates made ofMonel, a commercial alloy, meet these requirements very satisfactorily.The cathode plates are carried by wire stirrups 46, preferably of thesame metal, which are removably sup rted from metal angle members 47fastened to the inner faces of side walls 33, and having vertical drillholes evenly spaced `between the anode plate grooves 3 6 to take thehooked ends 49 of the stir-rups. The cathode plates are thuselectrically connected'together by these metal angle members 4"(` andthe current is supplied to the plates by connecting a generator acrossone such angle member 47 and the anode connecting strip 40. The spacingbetween anode and cathode plates may preferably,'though not necessarily,be about threequarters of an inch. With such spacing, and

the other proportions indicated, an applied l i potential actos theplates of .7 volt has been oundto provide eicient conditions.

The discharge end 32 of the trough has an overflow ed leading to dischare chute 17, which eege is located slightly a ve the upper edges of theelectrode plates so that the hypo may low both above and below theplates onvits, way through the trough.

Secured to the upper edges of slde walls 33 are air pipes 60 suppliedwith air from a common supply pipe 61, pipes being provided -with shortoutlet tubes 63 connected, as by rubber connecting tubes 64, with glassdischarge tubes 65 extending well down between the plates. The dischargetubes 65 are made of .glass rather than of metal for the reason thatmetal tubes would be attacked by the hypo solution. There are referablyat least one outlet tube from each of the two side pipes 61 dischargingbetween each pair of plates, and sufiicient airis passed upwardl toagitate-the hypo thoroughly, a copious fyoam appearing on the surface ofthe hypo when the action is proceeding roperly.

. This agitation is `found to very necessary to permit the concentrationto be reduced to as low as .2 oz. per gallon, as the silver does notreadily deposit from hypo to such a complete extent under ordinaryplating methods, and for the further reason that without agitation thesilver precipitates from the solution as silver sulfide.

It may now be seen that we have provided a systemv and method formanipulating the hypo solution by ,which the 'hypo is maintained at aconstant and extremely low silver concentration, and that the fixingtreatment is thereby rendered constant, variation of hypo strengthhaving been eliminated, and that large losses of silver due to carryingover relatively saturated hypo on the film are greatly reduced. And itwill further be seen that we have provided means lfor promoting theelectro-deposition of silver whereby the silver concentration may bereduced to lower values than have heretofore been possible.

1. The method of recovering silver from a silver laden photographicfixing solution which comprisesimpressin'g an electrical potentialacross an'anode and cathode in said solution, said anode being of asubstance st ..1 solution to move the same over the surface of saidcathode whereby metallic silver is deposited upon said cathode and theprecipitation of silver sulfide is prevented.

3. The method of recoverin silver laden "photographic fixing solutionwhich comprises impressing an electrical posilver from a tential acrossan anode and cathode in said solution, said anode being of a substanceother than silver, and bubbling air through said solution forcirculation thereof over the surface of saidly cathode whereby metallicsilver is deposited upon said cathode and the `precipitation of silversulfide is prevented.

4. The method of recovering silver from a silver laden photographicfixing solution which comprises impressing an electrical potentialacross an .anode plate and a cathode plate immersed in said solutionandin spaced relation, said anodeJ plate being of a substance other 'thansilver,and passing a stream of air through the solution between saidanode and cathode plates for circulation of the solution over thesurface of said-cathode plate. 5. The method of continuously employing aphotographic fixinor solution which comprlses dissolving the s1lverhalides from photographic light sensitive material in a bath of thephotographic fixing solution, conducting the silver laden photographicfixing solution to an electrolytic cell, impressing an elec,- tricalpotential across an anode and cathode in the solution in said cell, saidanode-'being of a substance other than silver, agitating the silverladenfixing solution for circulation thereof over the surface of saidcathode to cause the deposition of metallic silver thereon, andreturning the de-Silvered fixing solution to the fixing bath.V

6. The method of continuously employing a photogra hic fixing solutionwhich comprises disso ving the silver halides from photographic lightsensitive material in a bath of the photographic fixing solution,conducting the silver laden photographic fixing solution to anelectrolytic cell impressing an electrical potential across an anode andcathode` in the solution in said cellz said anode being of a substanceother than silver, bubbling air Y through the silver laden fixingsolution for circulation thereof over the surface of said cathode-tocause the deposition of metallic silver thereon, and returning thesilver-free fixing solution to the fixing bath.

7 The method of continuously employing a photographic fixing solutionwhich comprises dissolving the silver halides from photographiclight-sensitive material in a bath of the photographic fixing solution,conducting the silver laden photographic fixing solution to anelectrolytic cell, impressing an electrical potential across an anodeand a cathode in the solution in said cell, said anode being of asubstance other than silver, agitating the silver laden fixing solutionfor circulation thereofover the surface of said cathode to cause thedeposition of metallic silver thereon, cooling the de-silvered fixingsolution to a proper temperature for zn'ng treatment and returning thecooled and de-silvered fixing solution to the fixing bath.

. 8. The method of continuously employing a photographic fixing solutionwhich com.- prlses dissolving the silver halides from photographiclight-sensitive material in a bath of the photographic xing solution,conducting the silver laden photographic fixing solution to anelectrolytic cell, impressing an electrical potential across an anodeand a cathode in the solution in said cell, said anode being of asubstance other than silver, agitating the silver laden fixing solutionfor circulation thereof over the surface of said cathode to cause thedeposition of metallic silverA thereon, cooling the de-silvered fixingsolution to a proper temperature for fixing treatment by indirect heatconductive contact with a coolin medium and returning the cooled,-

nd de-silvered fixing solution to the fixing ath.

9. An electrolytic unit for recoverin metallic silver from usedphotographic ing solutions comprising a tank adapted to contain saidsolution, a cathode plate and an 'anode plate vertically positioned inhorizontal spaced relation within said tank, said anode plate being asubstanceother than silver and means for agitating said solution forcirculation over the surface of said cathode plate.

10. An electrolytic unit for recoverin metallic silver from usedphotographic ing solutions comprising a tank a apted to contain saidsolution, a cathode plate and an anode plate vertically positioned inhorizontal spaced relation within said tank, said anode plate being of asubstance other than silver and agitatlng means for bubbling air throughthe solution for circulation thereof over the surface of said cathodeplate.

' In witness that we claim the foregoing we have hereunto subscribed ournames this day of Februa 1929.

l`RANK GARBUTT.

THOMAS M. INGMAN.

